PROJECT SUMMARY/ABSTRACT Deoxyribonucleic acid (DNA) is the biological molecule within cells that is responsible for transmitting and storing genetic information. Unfortunately, DNA can become damaged, jeopardizing the integrity of this information that is vital for preserving health. Agents that damage DNA, known as reactive oxygen species (ROS), are produced in the normal course of cellular respiration as cells make energy by converting oxygen into water. They can also be introduced by outside sources such as ionizing radiation and certain transition metals. The experiments proposed here will focus on examining a specific type of DNA damage called DNA base oxidation, where the structure of the DNA base, responsible for storing genetic information, is altered. This particular type of DNA damage has been implicated in causing cellular aging, cancer, and neurological disorders like Alzheimer?s disease and amyotrophic lateral sclerosis. The goal of this proposal is to investigate the effect of the spiroiminodihydantoin (Sp) lesion on the dynamics, packaging, and repair of cellular DNA. This highly-mutagenic lesion, which is formed when ROS react with guanine bases in DNA, produces the same types of mutations found in some human lung tumors, and it has also been correlated with the progression of color cancer in mice with infection-induced colitis. In order to understand how cells detect and repair these kinds of DNA lesions, it is important to examine their base pair opening dynamics. Nuclelar Magnetic Resonance (NMR) techniques will be used in Specific Aim 1 to measure the base pair opening dynamcs for the Sp lesion, allowing us to better understand how this lesion is located by DNA repair proteins. The packaging of Sp lesions in nucleosomes, the most fundamental unit of chromatin, will be examined in Specific Aim 2 with the goal of providing information about how these lesions affect nucleosome assembly. Finally, in Specific Aim 3, the ability of base excison repair proteins to excise the Sp lesion from nucleosomes will be investigated. In sum, the results obtained from these studies will provide important insight into the biological processing of the Sp lesion in cells and will allow us to better understand how it may contribute to causing diseases like cancer.